The nuclear magnetic resonance image formation, it is necessary to place a body, sections of which it is desired to form as images, inside a continuous, intense and homogeneous magnetic field produced by a magnet. To improve the qualities of homogeneity of the magnet, it is necessary to know its inhomogeneity. The inhomogeneity of a magnet is measured by recording the values of the magnetic field produced at different positions of the volume concerned thereof. For this, a probe is moved to these different positions: the volume concerned is explored. It is known, in particular in magnetic resonance image formation, that the field must be homogeneous with a homogeneity of the order of a few parts per million (ppm). Therefore the positions in the inner volume where the field values are recorded must be located geometrically with as high a precision as possible. It also happens sometimes that the measurements must be reiterated, at the same positions of the magnet, after field corrections have been made. It is therefore necessary, besides the precision, to have equipment which is easy to handle. Finally, since the knowledge of the field produced by a magnet is all the higher the greater the number of points at which the field values have been able to be recorded, it is necessary for the exploration device to be simple in use.